This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Corrinoid-dependent enzymes are widespread in nature and play key roles in human, animal and microbial metabolism. Two known B12-dependent enzymes exist in humans: methylcobalamin (MeCbl)-dependent methionine synthase and adenosylcobalamin (AdoCbl)-dependent methylmalonyl-coenzyme A mutase. The clinical hallmarks of B12 deficiency are megaloblastic anemia (?pernicious anemia?), and/or neuropathies. Upon reaching cells, cobalamin derivatives are converted to MeCbl and AdoCbl by currently ill-defined mechanisms. Many studies have been carried out over the past several decades involving the extraction and identification of cobalamins from mammalian cells, tissue and blood, in addition to other biological samples such as foods and seaweed. Adenosylcobalamin, methylcobalamin and aquacobalamin are the major cobalamin metabolites isolated from biological samples but there are also reports of the isolation of nitrocobalamin and sulfitocobalamin from biological sources, along with other unknown complexes. We have synthesized a number of cobalamin derivatives to assist in the identification of unknown cobalamins isolated from biological samples, and we have determined the crystal structures of a number of these complexes including N-acetylcysteinylcobalamin, nitrosocobalamin, aquocobalamin, ethyelcobalamin, butylcobalamin and more recently glutathionylcobalamin all at atomic resolution (<0.8 A) using high energy x-rays (above 15 keV) and very short crystal-detector distances.